Something that commonly happens is that your brokerage sent you the cost basis for those sales at $0 on your 1099 form. There's typically an adjusted cost basis buried on their website which is what actually needs to be entered into your tax software as the cost basis, not $0.

So say you had $50k in stock vest and you sold $10k to cover the income tax. It's likely that when you import your 1099 that the cost basis is set to $0 on the $10k which if treated as a gain is a $2k-$3k tax bill. But if you find the adjusted cost basis and enter it the taxes drop to essentially $0.

So if you've experienced a huge, unexpected tax bill from RSUs vesting, go back and look at your tax return. If you see a $0 cost basis on the form then you overpaid. It may not be too late to amend your return and get it back.